Virtually every full-time or part-time four-wheel drive consumer vehicle, i.e., passenger car, truck or sport utility vehicle utilizes a transfer case to continuously or selectively direct a portion of the drive torque to a secondary driveline and drive wheels. Given the increased popularity of trucks and four-wheel drive vehicles, significant and increasing attention has been directed to the mechanical configuration of transfer cases.
One of the areas of development relates to the control of torque distribution in the transfer case and the myriad competing parameters which affect it. Torque distribution involves issues such as size of the transfer case, torque throughput, size of the clutch, desired operational modes, power consumption of the clutch, duty cycle of the clutch, power transfer demands of the vehicle, desired efficiency and many other physical and functional considerations.
One further consideration when torque distribution and throughput are analyzed is whether the secondary driveline torque must be modulated or whether it may be positively or directly provided such that the transfer case, in effect, achieves a 50--50, or other preselected, torque split between the two drivelines. If the latter is the desiderata, high torque throughput can be achieved with direct coupling but such a configuration then presents the problem of smoothly coupling the drivelines. Obviously, such coupling may occur when a vehicle is stationary, or nearly so, but on-the-fly coupling operation is much more desirable and, in the current market, virtually necessary.
The problem thus reduces itself to providing high torque throughput in a small, i.e., conventionally sized, transfer case also providing on-the-fly direct drive coupling.